It is standard as described in commonly owned U.S. Pat. No. 3,002,043 to carry a high-voltage conductor by means of an assembly of suspension and strut insulators. Each string of suspension insulators is paired with a strut insulator. One end of each string of suspension insulators and one end of each strut insulator are connected to a high-voltage transmission tower. The other ends are connected together to provide support for the conductor. The strings of suspension insulators are mounted at least partially in a vertical position and are designed to withstand tension loads from the weight of the conductor. The strut insulators are designed for compression loads and generally extend in or close to a horizontal plane from the tower. The suspension-insulator string and the strut insulator form a vertical plane perpendicular to the conductor.
In a case of uneven conductor spans or one span partially covered by a forest, hill, or the like, a wind blowing perpendicularly to a conductor will cause a longitudinally unbalanced load, causing in turn a bending of the strut. This uneven longitudinal tension of the conductor can be relieved by providing a horizontal hinge on the strut at the tower end. To provide a flexible way of stabilizing the longitudinal motion of the conductor, this assembly is equipped with two additional elements: a vertical hinge and a so-called stub arm. The latter is a rigid extension of the tower in line with the suspension insulator string. The outboard end of the strut insulator is connected to the end of the stub arm by a suspension string of a fixed length. As the strut swings horizontally to one side or the other, its outboard end rotates on the horizontal hinge around the end of the stub arm at a fixed distance causing it to lift up. Thus, the lifting up of the conductor provides a counterbalance to its longitudinal motion. Such insulator assemblies are commonly known as "horizontal vee" assemblies.
Another arrangement designed to support a high-voltage conductor while allowing some degree of longitudinal motion is a so-called "tie back." This is typically made of three strings of insulators: one horizontal compressive strut member and two strings of suspension insulators. The inboard end of the strut insulator is connected to a transmission tower through two hinges allowing the strut to rotate horizontally and vertically. The attachment points of the suspension strings to the tower are above the strut insulator, one on each side of the strut. The bottom ends of the suspension strings are tied together and support the outboard end of the strut insulator. The plane formed by both suspension strings is inclined to the horizontal. When longitudinally unbalanced conductor loads cause the strut to swing horizontally, its outboard end starts to rotate around the point of attachment of one of the suspension strings to the tower. The further the strut insulator swings horizontally, the higher its end is lifted up by its supporting suspension string. Thus the weight of the lifted conductor provides a counterbalance to its longitudinal movement.
When the strut insulator is in its balanced position, both suspension insulators pull its outboard end with equal forces in different directions. Due to the unbalanced conductor tensions when the strut swings horizontally, one suspension string loses its tension and the outboard end of the strut insulator is pulled to the opposite side by the other suspension string. Since the point of attachment of the suspension strings is not exactly in the longitudinal centerline of the strut insulator but somewhat off-center, this force creates a torsion on the strut. Since the offset arm is relatively short, this torsion is also small and is negligible.
In heavy-duty systems, two or more parallel strut insulators as well as suspension strings may be necessary to meet load requirements of the transmission line. As the outboard yoke plate increases in size to accommodate additional strut insulators, the distance between the vertical load from the conductor and the point of support of each suspension string increases considerably and increases torsion on the system. When an unbalanced longitudinal load moves the strut assembly laterally, the suspension of that side loses tension and all the vertical load is supported by only one suspension. This unbalanced unilateral support produces a high level of torsion on the system, that is, on the strut assembly, on the double-hinged inboard yoke plate between the struts and the tower, on the tower bolts, and on the tower cross arms.
The elimination of this torsional stress can be addressed in two ways. The first is by attaching the suspensions to the outboard yoke at a single point beyond the ends of the struts. This creates several problems. To avoid interference between suspension strings and a relatively large yoke plate, it is necessary to attach the suspension strings to the yoke plate at a location relatively far from the ends of the struts. This increases the effective length of the strut assembly and decreases its buckling strength. An increase in the effective length decreases the angle between the suspension strings and the strut assembly and puts an even greater compressive load on the struts. Both factors drastically limit loading capacity of the system. The second solution is shortening the struts. This decreases arcing distance and insulating capacity of the struts, a clearly unsatisfactory approach.